The IEEE standard 802.11a pertains to wireless local area networks (WLAN), and adopts Orthogonal Frequency Division Multiplexing (OFDM). OFDM is a technology that transmits data as multiple signals simultaneously over a single transmission path. OFDM spreads the data over a large number of carriers that are spaced apart at precise frequencies. Typically, a transmitter transforms frequency based data into the time-domain using an Inverse Fast Fourier Transform (IFFT) algorithm prior to transmission. A receiver then transforms a received packet back to the frequency domain using a Fast Fourier Transform (FFT) algorithm. The total number of sub-carriers translates into the number of points of the IFFT/FFT. In a wireless networking environment, OFDM has inherent advantages over a signal carrier system in a frequency-selective fading channel, such as high spectral efficiency, resiliency to RF interference, and lower multi-path distortion.
The IEEE 802.11 standard defines both an ad-hoc wireless network configuration and an infrastructure wireless network configuration that comprise a Basic Service Set (BSS). The ad-hoc configuration operates in a peer-to-peer mode that enables mobile stations to connect to each other directly, without the use of an access point. The infrastructure network configuration uses access points that bridge individual mobile stations with a wired network. Thus, receivers can be located at either individual mobile stations, or at access points to the network.
Under 802.11a, packets are mapped into a framing format suitable for sending and receiving user data and management information between two or more stations. This format includes a preamble field that includes a short preamble and a long preamble. The short preamble consists of 10 repetitions of a 16 sample short training symbol. The short preamble is used for received signal strength intensity (RSSI), automatic gain control (AGC), and coarse frequency synchronization. Subsequent to the preambles are a signal field, followed by multiple data fields. It is of critical importance to determine field boundaries as soon as possible in order to avoid a mismatch when processing the packet, and consequently, to avoid loss of the packet.
Two alternate methods of correlation have been used in connection with the preambles to determine field boundaries. An auto-correlation of the received signal involves correlating the signal with a delayed version of itself with the length of the correlation window equal to the length of a short symbol. When using an auto-correlation, a correlation plateau occurs due to the periodic nature of the short preamble. One approach to timing recovery is to look for the end of the correlation plateau in order to determine the field boundary.
A second approach uses a cross-correlation (or matched filter) between the received signal and the known values of the short symbol. Because the short preamble consists of 10 repetitions of a 16 sample training symbol, when the incoming data stream is correlated with the preamble, ideally the cross-correlation generates a peak every 16 samples, for a total of 10 peaks. This second approach looks to the last peak to identify the field boundary.